The majority of present day integrated circuits (ICs) are fabricated with a large number of interconnected field effect transistors (FETs), often called metal, oxide, semiconductor field effect transistors (MOSFETs or simply MOSTs). FETs can be either N-channel (NFET) or P-channel (PFET). Many ICs include both NFETs and PFETs. A FET includes a gate electrode as a control electrode and spaced apart source and drain regions formed in a semiconductor substrate and between which a current can flow. A control voltage applied to the gate electrode controls the flow of current through a channel between the source and drain regions. The gate electrode is electrically insulated from the underlying channel by a gate dielectric.
In the past, the gate electrode was typically formed of polycrystalline silicon (poly) and the gate dielectric was thermally grown silicon dioxide. To improve device performance, today many IC transistors use a high dielectric constant (high-k) insulator as the gate dielectric and several layers of metal or other conductive material as the gate electrode (a high-k metal gate transistor). Regardless of whether the IC is a poly gate structure or a high-k metal gate structure, fabricating a high yielding IC requires a method that controls the threshold voltage, the minimum control voltage applied to the gate electrode to initiate current flow, of all transistors of the IC. Control of threshold voltage, including setting the threshold voltage to a particular value is difficult, especially setting and controlling the threshold voltage of high-k metal gate PFETs. One method has been to oxygen anneal the partially formed metal gate, high-k dielectric structure, but this method can result in oxygen diffusing through the partially formed metal gate to the dielectric layer. The dielectric layer typically consists of two parts, a thin base oxide and an overlying high-k layer. Oxygen diffusion can result in enhanced base oxide thickness through regrowth of the base oxide. The increased thickness results in a reduced dielectric constant because of the enhanced equivalent oxide thickness. Additionally, the annealing temperature required for this process can be incompatible with the thermal budget for replacement metal gate technology.
Accordingly, it is desirable to provide methods for fabricating integrated circuits that control the threshold voltage, especially the threshold voltage of PFETs in the IC. In addition, it is desirable to provide methods for fabricating integrated circuits that are high yielding and are compatible with replacement metal gate integration. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.